Fluid mixing is a very important factor that needs to be considered in the design of bioreactors. Growing cells in bioreactors require sufficient amounts of nutrients and oxygen for their growth. The transfer of these nutrients to the cells can be ensured by sufficient mixing, yet excessive shear can destroy the cells. This study compares two impellers used for mixing: the “Dumbo Ear” impeller and the high efficiency A310 impeller.
Mixing time is used as the criteria of the performance of the impellers. Mixing time is defined as the time taken for the disappearance of the pink color of Phenolphthalein indicator in a basic solution by the rapid addition of acid. A known amount and concentration of sodium hydroxide is added to a bioreactor containing distilled water with a known amount phenolphthalein, turning the solution pink. Using pressure driven dispensing system (Figure 1), excess hydrochloric acid is rapidly injected to neutralize the base. The time it takes for the solution to become colorless is recorded as the mixing time. The mixing time is measured as a function of rotational speed for different locations of the impeller. The results are reported in figures 2–6.
Figure 2 compares Dumbo Ear impeller with the A310 at a clearance-to-height ratio (C/H) of 0.44 for a vertically centered shaft. In all the cases studied, the Dumbo Ear impeller out-performed the A310. The effect of varying clearance of the impeller on the mixing time was also investigated. Unlike the A310, the Dumbo Ear impeller showed better mixing as the clearance is reduced (Figure 3). In studying the effect of shaft angle on mixing time, two ways of pivoting the impeller were utilized. In the first case the impeller was pivoted at the intersection of the tank centerline with the top surface of the liquid. The impeller was also pivoted so as to maintain the bottom of the impeller shaft at the centerline of the tank. Thus in one case the impeller shaft entered the liquid at the centerline of the tank, and in the other case it did not, but the base of the shaft remained at the centerline. The shear forces will be different in the two cases because the clearances will be different. Figure 4 shows the variation in mixing time for a 15° shaft angle. In most of the cases studied, pivoting at the surface of the liquid gives a shorter mixing than pivoting at bottom of the impeller shaft. Figures 5 & 6 compare the mixing times for vertical and pivoted impellers for the two types of pivoting. In both types, tilting impeller reduces the mixing time.
This mixing study showed that in most cases, at the same rotational speeds, the Dumbo Ear impeller gave shorter mixing times than the A310. Also, tilting the Dumbo Ear impeller and having a small bottom clearance gave the shortest blending times. However, in all cases, one has to consider the increased shear forces on cells when clearance between the blades to the walls is reduced.